(a) Field of the Invention
The present invention relates to a metering separator adapted to first separate off the gas from a stream containing both gas and liquid, and then meter the liquid to provide a mass flow rate thereof.
More particularly, the assembly comprises: means for separating and removing the greatest part of the gas from the stream; means for accumulating the de-gassified liquid; means for monitoring the increasing weight of the accumulating batch; means for dumping the batch when its weight reaches a pre-determined value; means for monitoring the time taken to accumulate the batch; and means for computing the mass flow rate of the liquid from the weight and time information so derived and displaying the results.
(b) Prior Art
The present invention has been developed in connection with monitoring the production from wells in an in situ combustion project. While the invention is expected to find application as a meter outside this particular type of operation, the problems associated with metering combustion project streams will be discussed below, to illuminate the qualities sought in the development of the invention.
Combustion project production wells produce streams containing a mixture of gases, oil, water and solids. The proportions of these components vary constantly and over quite wide ranges.
The wells frequently are designed to produce from both the casing annulus and the tubing. The annulus stream is usually mainly gas, but can contain substantial amounts of liquid. The tubing flow is usually liquid but can contain substantial amounts of gas. It is desirable to meter the total combined gas content of both streams and the total combined liquid content of them.
As the annulus stream is usually mainly gas, it will create operating difficulties if introduced into most separators. Thus this stream is commonly routed directly to the flare line and, in many cases, no measurement of its quantity or rate is made.
The "liquid fraction" in the two streams commonly comprises viscous emulsions, which comprise oil, water and gases. Volume measurement is therefore ineffective, because of the unknown quantity of contained gas. In addition, flow of the viscous liquid fraction through pipelines and meters is complex; laminar flow and globular flow cause differential flow velocities of the various components (free water, free gas and emulsion), so that one velocity measurement of the fluids in a line or meter is meaningless, even if the density of the mixture could be determined.
It is desirable to meter this gas content in the production. Therefore it is a preferred object of this invention to provide a separator adapted to accept and meter both the casing and tubing flows at the same time.
One prior art device which has found commercial application in connection with metering the tubing stream of an in situ combustion production well can be referred to as a pivoting bucket meter. This meter involves a V-shaped container having two side-by-side, open-topped compartments. The container is pivotally mounted at its base, so that each compartment can tip back and forth between fill and discharge positions. In the fill position, one of the compartments is positioned beneath the outlet of the production flowline. In the discharge position, the filled compartment is tipped to dump its contents. While one compartment is filling, the other is discharging. The container is counterbalanced in such a way that it requires the accumulation of a certain weight of fluid in the compartment before the container will pivot.
In the use of the pivoting bucket meter, the number of dumps, occurring in a certain time period, are counted. The mass flow rate of the liquid can be approximated by calculations based on the weight and time data so obtained.
The pivoting bucket meter has been associated with certain problems, when used in connection with the production from an in situ combustion project. These problems mainly arise from the relatively high concentrations of gas in the production stream. When the stream is de-pressurized, by discharging into the open-topped compartment, much foam is generated. The possibility then exists that the production will overflow the compartment, without enough weight having been accumulated to pivot the compartment to the discharge mode. This of course deleteriously affects the metering operation and creates an undesirable spill.
The patent prior art discloses the concept of first separating the gas from the production stream and then metering the residue liquid. This is, for example, disclosed in U.S. Pat. No. 2,936,622, issued to Glasgow. The Glasgow reference is of interest because it teaches centrifuging the production stream in an upper chamber, to separate gas from the liquid. The gas is vented through a top outlet. The residue liquid then passes through a transfer line into a second chamber positioned beneath the first. Here the liquid is accumulated until it contacts and raises a float. The movement of the float initiates the closing of a valve in the transfer line and the opening of a dump valve in an outlet line from the second chamber. When the liquid has substantially drained from the second chamber, a second float at the base of the chamber is lowered and causes reversal of the valves.
In summary, the Glasgow unit couples gas separation with float-controlled volume metering.
Separators which use floats as the controlling means have been found wanting when used to meter in situ combustion project streams. This is because the gas in the emulsion is difficult to remove completely and foam is still present in the metering chamber. This foam will activate the float prematurely and result in an inaccurate reading of the true liquid volume being passed.
A third approach to metering this type of production involved simply producing it into a storage tank and timing the accumulation. The production then is held in the tank long enough to allow the bulk of the gas to break out and be vented. The residue is then measured and centrifuging of samples will give a breakdown of the oil, water and solids. With this information, the mass flow rate of the oil can be calculated.
However, while accurate, this type of metering yields data that may be several days old. It is preferable that the information be as current as possible, for purposes of analyzing well pumping problems and understanding what may be taking place in the sub-surface reservoir.
There is thus still a need for a device capable of accurately monitoring the liquid content mass flow rate of a gassy oil stream, such as the combined casing and tubing production of an in situ combustion project production well.